Composite tape strips are used in the aerospace and other industries to form composite parts. The demand for composite parts is increasing as new uses for such parts are explored. As the demand for composite parts has increased, it has become more desirable to efficiently automate composite part manufacturing methods. Because composite parts are often used in safety critical environments, such as in the manufacture of airplanes, the parts must be of a high and verifiable quality. The quality of composite parts depends in large part on the accuracy of the tape lay-up process and the quality of the material used.
A tape lay-up machine for forming composite parts generally includes a tape feed mechanism from which rolled tape is dispensed, a guide shoe for guiding the tape onto a receiving surface, and a pressure foot for smoothing the tape against the receiving surface. The tape lay-up machine also includes a control system. The tape is laid according to a preprogrammed numerical control (NC) path plan, which is executed by the control system. The tape lay-up machine moves in three dimensions in relation to the part that is being formed; the lay-up machine also has a warp axis for laying of contoured shapes. The tape strips are laid side-by-side, one strip at a time to form a layer, and one layer at a time. Generally, the NC path plan is nonadaptive; the path plan is not modified during the lay-up process in response to inconsistencies in the tape width and/or tape skew. If a layer does not meet the requisite quality requirements, the layer is removed and a portion of the NC path plan is rerun in order to re-lay the layer.
One major quality control focus in tape lay-up processes is on the minimization of gaps and overlaps (negative gaps) between adjacent tape strips in each ply. The existence of gaps/overlaps between the tape strips affects the integrity of the composite part. Gaps and overlaps between strips may be caused by the skewing of one or both of the tape strips from their expected centered position and/or by variations in the tape width. Skewing of the tape may result from a poorly rolled tape source or from the lay-up process itself. Quality control during a tape lay-up process normally includes visual inspection of each layer for gaps/overlaps. If the gaps and overlaps in a layer are out of tolerance, the tape strips are removed and new strips are laid. This type of quality control may provide an acceptable final product, but it does not provide accurate information regarding the cause of the gaps and overlaps. Thus, it is difficult to determine whether problems are caused by poor tape quality (e.g., width or roll-up inconsistencies) or by the operation of the tape lay-up machine. Such a quality control procedure is also one reason why tape lay-up processes are very labor intensive.
In order to accurately monitor the gap/overlap occurring during the tape lay-up process and to provide useful information about the quality of each layer, the width of the tape and the tape's centerline skew are monitored. If the tape width is greater or less than expected, overlaps and gaps, respectively, may result. If the centerline of the tape, as it is laid, is skewed from the expected centerline, an overlap may occur at one edge of the tape and a gap at the other edge. A combination of tape width inconsistencies and centerline skew can create significant gaps and overlaps between adjacent tape strips.
Many systems exist for monitoring the width of strips. For example, when steel, glass and tape strips are produced, the quality of the strips will depend in part on the consistency of the strip's width. Width monitoring systems are often integrated into these types of strip manufacturing systems. In certain width monitoring systems, the positions of both edges of the strip are monitored in order to calculate the width of the strip. In other width monitoring systems, the position of one edge of the strip is monitored; the second edge is aligned against a fixed barrier. In these strip manufacturing systems, the relative position of the material strip on a receiving surface is not considered. Contrariwise, the systems are generally designed to monitor the strip width while limiting the effects of strip skewing on the monitoring process.
An example of a strip width monitoring system is the luminous object monitoring system disclosed in U.S. Pat. No. 4,033,697 (Pfoutz et al.). The system monitors the width of a hot strip of material during manufacture by using two sensing devices placed above the strip and spaced apart a distance to permit viewing of each edge of the strip. The sensing devices each include an array of light-sensitive detectors. The strip edges are imaged onto the detectors using visible and infrared radiation from the hot strips. Edge position information is determined for each edge. The edge position information is combined with the fixed dimension between the two sensing devices. The result represents the total strip width. The strip width information is provided to an operator for manufacturing control purposes. The strip width information is generally relatively inexact and is not used for analysis of strip position.
Prior systems also exist for monitoring the centerline of a strip of material. An example of such a system is a system for controlling the rolling of tape onto a spool. Many centerline monitoring systems use edge detection techniques similar to those described in the Pfoutz et al. patent. Such systems generally test the tape edge positions to determine whether the edges are symmetrically positioned about a desired centerline. Such systems do not provide centerline data or centerline data analysis. These systems are used in applications wherein the centering of the tape is important but the relative position of the tape on a receiving surface is not of consequence. In such applications, the tape width is also not of consequence.
One drawback of prior strip width detecting and strip centering systems is that the systems are relatively large. For example, many prior systems include backlighted areas requiring one or more illuminating components arranged below the surface over which the strips travel. The remainder of the monitoring system is arranged above the surface. Such systems cannot be incorporated into a tape lay-up machine near the pressure foot because of size constraints. Another drawback of prior systems is that they do not generally provide accurate strip width and centerline skew information that can be analyzed to provide a measurement of gaps and overlaps between adjacent strips on a receiving surface. The present invention provides solutions to these and other problems in the prior art.